Recently, ceramic materials have been under development in view of the heat resistance thereof. In many cases, heat-resistant ceramic materials have found main use in the peripheral parts of engines. Among others, it has practically been desired to make the turbine blades used with gas turbine engines, turbochargers, etc. of ceramic materials due to increase in the operation temperature and decrease in the weight thereof and, hence, an improvement in the response characteristics thereof.
In practical use, so high is the temperature of the joined turbine blades that attention should be paid to the insulation of heat transmitted therefrom in view of the durability of the associated bearing parts and lubricating oils.
Turning now to FIG. 1, there is shown the basic structure of a typical gas turbine (or a turbocharger). As well-known in the art, a casing shown generally at 1 is constructed from three casing members, the first being a turbine casing 2 including therein an inlet and an outlet both for exhaust gases, the second a compressor casing 4 including therein an air suction port and an air discharge port, and the third being a bearing casing interposed therebetween. A turbine rotor 5 extends axially through the central portions of the three casings. A hot exhaust gas discharged from an engine, etc. is introduced into the casing 2 through an inlet 21 to rotate a turbin disc 51 (usually made of ceramic material) at a high speed, and is discharged therefrom through a central gas outlet 23. A shaft 53 formed of a ceramic material solid with the turbin disc 51 and a metal shaft 53 joined thereto are journalled and axially positioned by the bearing casing 3, so that the turbine disc 51 rotates under the lubricating action of an oil supplied from an oil port 31, thereby to rotate at a high speed a compressor disc 54 housed within a compressor disc chamber 41 and fixedly fitted over the metal shaft 53. Air is sucked through an air suction port 42, is compressed in the disc 54, and is supplied into an internal combustion engine, etc. through the compressor casing 4.
In the case of currently used metal turbine blades, on the other hand, a void 64 for heat insulation is defined in both the end 62 of the turbine rotor 62 formed of a heat-resistant alloy and the end of a metal shaft 63 joined thereto with the contact surface of both being welded together, as will be understood from FIG. 2(a) illustrative of the junction structure. Since the metal shaft 63 is practically cooled by an oil thus heat is primarily transferred through the central portion of that shaft, it may be said that such a void 64 well performs its insulating function due to it's central location.
In the case of ceramic turbine blades solid with which the associated rotor and shaft are formed of ceramics, however, it is difficult to apply the heat insulation structure similar to that applied to the metal turbine blades, since the coefficient of thermal expansion of ceramics is lower than that of metals.